linux-kernel - Re: [PATCH RFC 00/10] RDMA/FS DAX truncate proposal

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Date:   Wed, 12 Jun 2019 11:41:53 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Jan Kara <jack@...e.cz>
Cc:     Jason Gunthorpe <jgg@...pe.ca>, Ira Weiny <ira.weiny@...el.com>,
        "Theodore Ts'o" <tytso@....edu>, Jeff Layton <jlayton@...nel.org>,
        Dave Chinner <david@...morbit.com>,
        Matthew Wilcox <willy@...radead.org>,
        linux-xfs <linux-xfs@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        John Hubbard <jhubbard@...dia.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        linux-ext4 <linux-ext4@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>
Subject: Re: [PATCH RFC 00/10] RDMA/FS DAX truncate proposal

On Wed, Jun 12, 2019 at 5:09 AM Jan Kara <jack@...e.cz> wrote:
>
> On Wed 12-06-19 08:47:21, Jason Gunthorpe wrote:
> > On Wed, Jun 12, 2019 at 12:29:17PM +0200, Jan Kara wrote:
> >
> > > > > The main objection to the current ODP & DAX solution is that very
> > > > > little HW can actually implement it, having the alternative still
> > > > > require HW support doesn't seem like progress.
> > > > >
> > > > > I think we will eventually start seein some HW be able to do this
> > > > > invalidation, but it won't be universal, and I'd rather leave it
> > > > > optional, for recovery from truely catastrophic errors (ie my DAX is
> > > > > on fire, I need to unplug it).
> > > >
> > > > Agreed.  I think software wise there is not much some of the devices can do
> > > > with such an "invalidate".
> > >
> > > So out of curiosity: What does RDMA driver do when userspace just closes
> > > the file pointing to RDMA object? It has to handle that somehow by aborting
> > > everything that's going on... And I wanted similar behavior here.
> >
> > It aborts *everything* connected to that file descriptor. Destroying
> > everything avoids creating inconsistencies that destroying a subset
> > would create.
> >
> > What has been talked about for lease break is not destroying anything
> > but very selectively saying that one memory region linked to the GUP
> > is no longer functional.
>
> OK, so what I had in mind was that if RDMA app doesn't play by the rules
> and closes the file with existing pins (and thus layout lease) we would
> force it to abort everything. Yes, it is disruptive but then the app didn't
> obey the rule that it has to maintain file lease while holding pins. Thus
> such situation should never happen unless the app is malicious / buggy.

When you say 'close' do you mean the final release of the fd? The vma
keeps a reference to a 'struct file' live even after the fd is closed.